$gtf 1024 768 75

# 1024x768 @ 75.00 Hz (GTF) hsync: 60.15 kHz; pclk: 81.80 MHz
Modeline "1024x768_75.00" 81.80 1024 1080 1192 1360 768 769 772 802 -HSync +Vsync

$

Generalized Timing Formula is a standard by VESA which defines exact
parameters of the component video signal for analog VGA display
interface.

The parameters defined by the standard include horizontal blanking
(retrace) and vertical blanking intervals, horizontal frequency and
vertical frequency (collectively, pixel clock rate or video signal
bandwidth), and horizontal/vertical sync polarity. These parameters are
used by the XFree86 Modeline, for example. This standard is available from VESA for $350. [1]

The standard was adopted in 1999, and was superseded by the
Coordinated Video Timings specification in 2002.

Reference :

http://en.wikipedia.org/wiki/Generalized_Timing_Formula